Blank for shadow mask for color television picture tube

ABSTRACT

The shadow mask of a color picture tube is formed by etching a blank to provide a field of apertures individually having a large diameter portion and a coaxially aligned, small diameter portion attached to the former by a thin wall section. In screening a tube which is to utilize such a shadow mask for color selection, the phosphor materials are deposited on the screen in a photographic process involving the exposure of a layer of photosensitive material by actinic energy directed through the small diameter portions of the mask apertures. After screening has been accomplished, the thin wall sections are etched away, leaving the mask with apertures of large diameter, larger than the phosphor deposits on the screen.

United States Patent 1191 Black BLANK FOR SHADOW MASK FOR COLORTELEVISION PICTURE TUBE [75] Inventor: Joseph M. Black, Chicago, Ill.

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

[22] Filed: Oct. 7, 1971 [21] Appl. No.: 187,487

Related US. Application Data [62] Division of Ser. No. 850,408, Aug. 15,1969, Pat. No.

52 us. c1. 29/190, 313/92 B 511 Int. Cl. B21c 37/00 [58] Field of Search29/190; 313/92 13, 313/85 s; 96/361 [56] References Cited UNITED STATESPATENTS 3,574,013 4/1971 Frantzer 313/92 B 3,666,462 5/1972 Kaplan313/85 S Aug. 21, 1973 2,750,524 6/1956 Braham 96/36.1 X

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-M. J. AndrewsAttorney-John .I. Pederson and Nicholas A. Camasto [S 7] ABSTRACT Theshadow mask of a color picture tube is formed by etching a blank toprovide a field of apertures individually having a large diameterportion and a coaxially aligned, small diameter portion attached to theformer by a thin wall section. In screening a tube which is to utilizesuch a shadow mask for color selection, the phosphor materials aredeposited on the screen in a photographic process involving the exposureof a layer of photosensitive material by actinic energy directed throughthe small diameter portions of the mask apertures. After screening hasbeen accomplished, the thin wall sections are etched away, leaving themask with apertures of large diameter, larger than the phosphor depositson the screen.

5 Claims, 5 Drawing Figures BACKGROUND OF THE INVENTION One of the mostattractive developments in the shadowmask color television tube is theblack surround screen described and claimed in US. letters Pat. No.3,146,368, issued on Aug. 25, 1964, and assigned to the assignee of thepresent invention. The principle of black surround is applicable whetherthe phosphor deposits comprising the screen are in the form of strips,hexagons or dots. The configuration of the deposit is of no particularconsequence but, for convenience, attention will be directed initiallyto the dot triad screen structure having black surround. Such a screendiffers from antecedent devices in that the phosphor triads have dots ofvarious phosphors which are smaller in diameter than the apertures ofthe shadow mask, whereas in earlier devices the phosphor dots of thetriads are larger in diameter compared to the apertures of the mask andare in essentally tangential contact with one another. By reducing thediameter of the phosphor dots, there is provided a separation betweenthe dots to which may be applied a light-absorbing materal or pigment.Such a material is usually black and surrounds the phosphor dots fromwhence its name derives.

It will be apparent on reflection that such a tube presents problems inscreening; in particular, it imposes the condition that the phosphordeposits be smaller in dimension than the apertures or electrontransparent portions of the shadow mask which is also referred to as thecolor-selection electrode.

Screening a tri-color tube is most easily accomplished by photographictechniques in which the image area of the tube is covered with a lightsensitive material that is to be exposed by actinic energy directed tothe screen through the apertures of the shadow mask. Such an approach isbeneficial in that it accomplishes precision in the positioning of thephosphor deposits on the image area of the tube in relation to theapertures of the shadow mask. Generally, the mask is in such juxtaposition to the screen that the phosphor deposits, de-

termined through such an exposure, are about the same in configurationbut larger in dimension than the apertures of the mask. Therefore, somedeviation from normal screening practices is required to satisfy thecondition of the black surround tube that its phosphor deposits besmaller in size than the mask apertures.

A variety of approaches to this problem have been suggested,representative ones being the subject of U. S. letters Pat. No.3,070,411, issued on Dec. 25, 1962 and U. S. letters Pat. No. 3,231,380,isued on Jan. 25, 1966. These earlier efforts contemplate that the maskwill be formed in conventional fashion with apertures of normal size.The apertures are then stepped down or reduced in size on a temporarybasis for the purpose of having proper dimensions for screening. To thatend, the holes are temporarily filled with a material, usually a metaldifferent from that of the mask blank, which is removed after screeninghas taken place. The filler may be removed, for example, by etching. Intheory, this is an acceptable solution but raises practical problemsbecause the filler is generally applied while the mask is still in sheetor planar form and when it is pressed into its required dome shape,difficulties of cracking or distortion are frequently experienced.Similar problems giving rise to imperfections in the phosphor deposits,can be expected in most processes in which the mask is formed with largesize apertures that are to be stepped down by some temporary scheme ofclosing or filling the apertures. The present invention is a differentsolution to this problem which avoids difficulties of the earlier artand permits better control and uniformity of both the mask apertures andphosphor deposits.

Accordingly, it is an object of the invention to provide an improvedshadow mask to be used for the fabrication, and in the operation, of ashadow mask type of color picture tube.

SUMMARY OF THE INVENTION A shadow mask for a shadow-mask type of colorpic ture tube embodying the invention comprises a metallic sheet havinga field of apertures which individually have a first portion of a givenconfiguration (a slot or circular hole depending on the shape desiredfor the phosphor deposits) and a second but smaller portion of the sameconfiguration. The second portion is disposed in nested relation to thefirst portion and is attached thereto by a wall section having athickness which is but a small fractional portion of the thickness ofthe sheet. For the dot triad type of tube the aperture portions arecircular and the interconnecting wall section is provided by an annularrecess that is concentric with the small diameter portion of each maskaperture.

BRIEF DESCRIPTION OF THE DRAWING The features of the present inventionwhich are believed to be novel are set forth with particularity in theappended claims. The invention,together with further objects andadvantages thereof, may best be understood by reference to the followingdescription takenv in connection with the accompanying drawing, in theseveral figures of which like reference numerals identify like elements,and in which:

FIG. 1 is a fragmentary view of the screen section of a shadow-mask typeof color picture tube and also of a shadow mask in screening relation tothe image area of the tube;

FIG. 2 is a view on an enlarged scale of one of ertures of the maskrepresented in FIG. 1;

FIGS. 3 and 4 are views, likewise on an enlarged scale, of another formof aperture that may be provided the apin the shadow mask; and

FIG. 5 represents still another form of mask or colorselection electrodeembodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Color tubes of the type underconsideration may have circular or rectangular envelope configurationsat the faceplate or screen section which defines the image area. In thisarea, there is applied a repeating sequence of phosphors representingthe three primary colors because commercial television as presentlypracticed is an additive type of system. The phosphor deposits may bestrips, hexagons, circular dots or the like, as stated above. Thespecific configuration of the envelope or of the elemental phosphordeposits is ofjno particular consequence to the present inventionbutforconvenience it will be assumed that the tube under process is arectangular one having a mosaic screen defined by a multiplicity of dottriads with each such triad including a dot of green phosphor, a dot ofblue and a dot of red phosphor. 1n the fragmentary view of FIG. 1, thecircles denote phosphor deposits and the legends G, B and R symbolicallyrepresent those of the deposits which are formed of green, blue and redphosphors, respectively. It will be observed that the phosphor dots aresmaller than usual and, therefore, are not in tangential contact;instead, they are separated from one another and the area of the screenthat surrounds each such dot may accommodate a light-absorbing materialin accordance with the teachings of U. S. letters Pat. No. 3,146,368,although for convenience of illustration, that pigment has not beenrepresented in the drawing. Moreover, a shadow mask tube featuringpost-deflection-focus employs the same screen arrangement whether or notit also has black surround.

Superposed over and in close spaced relation to the screen or image areaof the tube is a shadow mask 10, only a portion of which is shown inFIG. 1. It is a relatively thin metallic sheet, usually formed from coldrolled steel having a thickness of approximately 5 mils. This sheet hasa multiplicity of apertures 11 arranged in a field with dimensions andconfiguration corresponding to the dimensions and configuration of theimage area. As shown in the enlarged view of FIG. 2, each individualaperture of the mask has a large diameter portion 11a of generallyconical shape having a maximum diameter D Each aperture of the fieldalso has a coaxially aligned, small diameter portion 11b of much thesame shape and with a maximum diameter D The dimension D represents thepart in common to portions 11a and 1 lb or the area over which the oneopening is in communication with the other. The smaller diameter portion11b is attached to large diameter portion lla by a wall section 11dhaving a thickness t that is a small fractional portion of the thicknessof the sheet from which the mask has been made. So long as the aperturesof the mask are constructed as described and as illustrated in FIG. 2,they present an effective aperture having a dimension D chosen to theend that phosphor dots applied to the image area of the screen in aphotographic technique, considered hereinafter, are of a desired size.

Before considering the screening process, as such, it is appropriate todescribe a method of forming a shadow mask having apertures of the typeillustrated. It is most convenient to use a metal stock, such as coldrolled steel, into which apertures may be formed, as by etching, withprecision not only as to their location within a desired field but alsoas to dimension. Procedures for accurately and controllably etchingapertures in such a metal sheet are totally within the skill andknowledge of those engaged in etching of sheet metal. The generalprocess, for example, is described in the following US. letters Pat.Nos: 2,750,524 issued on June 12, 1956; 2,762,149 issued on Sept. 11,1956; and 2,961,313 issued on Nov. 22, 1960.

By and large, the process involves well-known techniques ofphotoprinting by which a pattern is established on a sheet followed bychemical milling or etching through which the pattern is developed. Moreparticularly, in considering the aperture structure of FIG. 2, the maskblank is first coated on both sides or surfaces with a photosensitivematerial having such properties that its. solubility in a given solventis influenced by exposure to actinic energy. Preferably, an organicphotosensitive material is employed of such nature that those portionsthereof that are exposed to ultraviolet light are rendered insoluble.Having applied coatings of this material to both surfaces of the blank,a design is projected on each face thereof by ultraviolet light totransfer that design by the exposure. Specifically, the pattern of thedesign for the mask under consideration shields each aperture portion11a on one surface of the mask blank from exposure and similarly shieldseach portion 11b on the obverse side of the blank. The design projectedon the same surface of the sheet as the smaller aperture portion 11b isfurther arranged to shield from exposure a concentric annulus or ring11c having an inner diameter which is less than the diameter D ofaperture portion and an outer diameter substantially equal to thedimension D,. Of course, the designs are projected from accuratelydetermined positions so that shielded portions of the photosensitivecoating representing aperture portions 1 la and 1 1b are in coaxialalignment throughout the aperture field. Aside from these shieldedportions associated with each aperture to be formed in the mask blank,the entirety of the blank is exposed on both surfaces and the imageresulting from such exposure is developed by spraying, washing orotherwise treating the surfaces of the blank with the solvent for thephotosensitive coating. In this treatment, the exposed portions of thephotosensitive material remain in situ while the rest of the coating iswashed off. As a consequence, the surface area of the blank at thelocation of each aperture portion that is to be formed as well as at thesitus of each concentric ring 1 1c is stripped of its coating and isthus exposed or laid bare. These exposed elemental areas of the maskblank are then subjected to an etch bath of ferric chloride of suitableconcentration. While the exposed elemental areas of each side of themask blank may be etched individually, it is preferred, as described incertain of the afore-identified patents, to etch both sides or faces ofthe mask blanksimultaneously.

The portion designated D, which is common to aperture portions 11a and11b is determined or controlled as to size by the diameters D, and D andthe etching time. Concurrently with etching out aperture portions Ila,and 11b, the thin wall section 1 1d is formed by removing most of thetransverse section of the mask blank intervening portions 1 la and 1 lbso that wall section 11d may, at a subsequent time, be removed withfacility, speed and accurate control. In etching out the recess 11c toform wall section 11d, the inner and outer diameters defining annulus 11c are dimensioned so that the annular recess has a depth h that is atleast equal to, but preferably exceeds, the depth In of the smalldiameter portion 1 lb. Controlling the depth of this recess determinesthe thickness 1 of the interconnecting wall section. As stated, it ismade as thin as possible while preserving sufficient mechanical strengththat the mask blank, having been provided with the described field ofapertures, may be domed or formed without destroying or breaking freethe small aperture portions 111;. A thickness of about 1-2 mil isbelieved to be acceptable.

The shadow mask with the field of formed apertures is appropriate foruse in screening the picture tube. In the first screening step, if thetube is of the dark surround variety, the image area of the tube iscovered with a photosensitive resist composition in which lightabsorbingmaterial, such as manganese carbonate, is

carried in suspension as described in U. 8. letters Pat. No. 3,365,292.This resist is of the type that it is rendered soluble upon exposure toactinic energy and the coated screen is exposed three times with eachexposure being made through the shadow mask. In this process step,actinic energy, such as ultraviolet light, is able to reach the screenarea only through aperture portions 11b so that each exposed area of thesensitized coating is a circle of dimension D,. In each of the threeexposure steps, the light source is positioned to simulate one of thethree electron guns of the tube and in this fashion, latent images areestablished of the elemental areas of the screen that are to receive thephosphor deposits. Washing the exposed screen with a solvent for thephotosensitive resist develops these images by removing all exposedportions of the coating. What remains on the screen is the blacksurround material as a suspension in an organic material that will beremoved in bakeout.

The screen is now in condition to receive the phosphor deposits and eachmaterial is applied by means of a similar photographic technique inwhich the image area is first covered with a layer of photosensitiveresist material carrying one of the three phosphors in suspension.lnthis instance, however, an opposite type resist is employed. That isto say, the resist is of the kind that is rendered insoluble by exposureto ultra-violet light. Polyvinyl alcohol sensitized by ammoniumdichromate is a very useful resist for this purpose since it is solublein water. After the exposure, latent imagesof the deposits of aparticular phosphor will have been made and they are developed bywashing the screen with a solvent, such as water, leaving deposits ofthe phosphor properly located on the screen area relative to theblack-surround material. In like manner, the remaining two phosphormaterials are applied but in each instance, the location of the exposinglight source is adjusted to simulate the assigned beam of the tube inprocess. At this juncture, the screen will have been completed so far asthe deposits of phosphor and the blacksurround material are concerned.It may now be aluminized in the usual way so that the phosphor dots havea backing layer of an electron permeable, conductive and lightreflecting metal, usually aluminum. After aluminizing, the screen isready for bakeout although this step is usually taken with the mask inposition within the faceplate section of the tube.

With the screening accomplished, the small diameter portion 11b of eachaperture of the mask may be removed and this is achieved by a secondetching process in which the wall sections 11d are etched outuHavingremoved aperture portion 11b, each aperture of the mask will have beenenlarged to the dimension D, which is controlled by the etching steputilized to remove wall sections 1 1d. The mask is now conventional,having apertures of normal size, and is suitable for securing into thefaceplate section of the tube in essentially spaced parallel relation tothe screen area, preparatory to bakeout.

A shadow mask constructed in accordance with the invention has distinctadvantages in fabricating a color tube. Frequently, the mask is made bya fabricator having equipment with suitable controls to accuratelyposition and dimension aperture portions 11a, 11b and recess 1 1c. Theaccuracy attainable, particularly as to aperture portion 11b, providesthe desired position, configuration and dimension of the phosphor dotsdeposited on the screen of the tube by exposing through such a mask.Distortions of the spot configuration and nonuniformities attributableto past techniques of temporarily filling in the apertures of the shadowmask are avoided. Another advantage is the simplicity with which a tubemanufacturer, having used the mask for screening purposes, may removeaperture portions llb and convert the structure to a mask with holesthat are larger than the phosphor deposits as required for ablack-surround or post-deflection-focus type of picture tube. Since thewall sections t are very thin, they may be etched away in a very shortperiod of time, in a few minutes, if desired, and without requiring thesophistication of control desired in originally etching and forming maskportions 11 a, 11b and He. Still another advantage results from the factthat the second or re-etch, being accomplished in a short period oftime, removes a minimal amout of the surface of the mask whichintervenes each of its apertures. This is desirable because otherwise itwould be necessary materially to increase the thickness dimension of themask blank with attendant disadvantages in processing times and materialcost.

If desired, the modified aperture structure of FIGS. 3 and 4 may beemployed. In this case, large diameter portions 11a and 11a are formedin opposed elemental areas on both surfaces of the blank leaving,partway in the thickness of the blank, the small diameter portion lllb.In this modification and as shown in FIG. 3, the thin interconnectingwall sections 11:! may be formed by etching out arcuate sections 11cdistributed about a path concentric with aperture portion 1112. Ifsections lle are etched through, it is desirable to fill them with anopaque material so that they do not occasion unwanted exposures duringthe screening process. Alternatively, these portions may be partiallyetched out, leaving a wall section lle' so that only the part 11b ofeach aperture is transparent to actinic energy.

Of course, the multiplicity of apertures in the shadow mask may beidentically dimensioned or, if desired, they may be weighted. Forexample, it is sometimes arranged that the diameter of the apertures arelargest at the center of the mask and decrease with radial spacing fromthe center in order to obtain a brightness increase at the center of thescreen and to provide an acceptable tolerance for beam registration atthe edges of the screen. This result may be realized in the practice ofthe present invention simply by photoprinting with an aperture patternthat has the hole weighting desired.

As indicated above, the specific configuration desired of the aperturein the mask is of no moment in practicing the invention and the enlargedfragmentary view of FIG. .5 shows the application of the invention to amask having elongated slots rather than circular holes. Morespecifically, each opening in the mask has a first portion 21a in theform of a slot of width D extending entirely across the mask blankexcept for its ends (one of which appears in FIG. 5). The ends areimperforate and border the series of slots 210. Each opening also has asecond but smaller portion 21b, likewise in the form of a slot of widthD, disposed on the opposite side of the mask blank in nested relation toportion 21a and attached thereto by narrow wall sections 21d formed byrecesses 211a etched out of the blank on opposite sides of each slot21b.

In using this mask, a line or strip type screen is appliedphotographically with the location and dimensions of each phosphordeposit determined by actinic energy projectd onto the screen surfacethrough slots 21b. After screening, the mask is re-etched to sever wallsections 21d and cause the screen to have only large slot-type apertures21a.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim: 7

l. A shadow mask for a shadow-mask type of color picture tubecomprising:

a metallic sheet having a field of apertures individually having a firstportion of a given configuration and a second but smaller portion formedby an annular recess of substantially the same configuration disposed innested relation to said first portion and attached thereto by a wallsection having a thickness that is a small fractional portion of thethickness of said sheet.

2. A shadow mask in accordance with claim 1, in which said first portionof each said aperture is formed in one surface of said sheet and saidsecond portion is formed on the opposite surface thereof.

3. A shadow mask in accordance with claim 2, in which said first andsecond portions are circular and in which said wall section isconcentric with said second portion.

4. A shadow mask in accordance with claim 3, in which the depth of saidfirst portion is at least equal to one half the thickness of said sheet;

and the depth of said recess is at least equal to the depth of saidsecond portion.

5. A shadow mask in accordance with claim 4, in which the depth of saidfirst portion is greater than one half the thickness of said sheet;

and in which the depth of said recess exceeds the depth of said secondportion.

a t t k

2. A shadow mask in accordance with claim 1, in which said first portionof each said apertUre is formed in one surface of said sheet and saidsecond portion is formed on the opposite surface thereof.
 3. A shadowmask in accordance with claim 2, in which said first and second portionsare circular and in which said wall section is concentric with saidsecond portion.
 4. A shadow mask in accordance with claim 3, in whichthe depth of said first portion is at least equal to one half thethickness of said sheet; and the depth of said recess is at least equalto the depth of said second portion.
 5. A shadow mask in accordance withclaim 4, in which the depth of said first portion is greater than onehalf the thickness of said sheet; and in which the depth of said recessexceeds the depth of said second portion.